It is well known in the art to convert olefins to aldehydes having one additional carbon atom by contacting the olefin with hydrogen and carbon monoxide in the presence of a catalyst based on cobalt or rhodium metal. Rhodium-based catalysts have the advantage, relative to cobalt-based catalysts, of being able to promote the hydroformylation of olefins under less severe operating conditions.
One disadvantage of prior art rhodium-based catalysts is the propensity of such materials to lose activity over a period of time as a result, for example, of ligand decomposition. Triaryl phosphines, for example, are prone to conversion into alkyl diaryl phosphines under hydroformylation reaction conditions. These alkyl diaryl phosphines as rhodium ligands give lower activity catalysts compared to the triaryl phosphines.
Another disadvantage of prior art rhodium-based catalysts is the fact that not all rhodium salts are suitable starting materials for the preparation of rhodium complexes. For example, it is frequently observed that a several hour induction period is required to transform the rhodium complexes into active hydroformylation catalysts. This problem is particularly acute when halide containing compounds of rhodium are employed for the preparation of rhodium complexes.
Yet another disadvantage of rhodium-based catalyst systems is the high cost of the rhodium metal employed for catalyst preparation. Where one employs low levels of rhodium metal in order to reduce catalyst costs, low reaction rates frequently result.
There is, therefore, a continuing need in the field for high activity, high selectivity rhodium-based hydroformylation catalyst systems.
There is also a continuing need in the field for selective catalyst systems for the hydroformylation of alpha-olefins. Catalyst systems which are tailored to prepare aldehyde products having specific linear to branched chain isomer ratios would be particularly valuable. Those of skill in the art recognize that there is substantial potential market for derivatives of branched-chain aldehydes, as well as the existing large market for linear aldehyde hydroformylation products.
Current commercial scale hydroformylation plants based on high pressure cobalt carbonyl catalyst systems produce marketable quantities of both linear and branched-chain aldehydes. The presently preferred, low pressure hydroformylation employing rhodium-based catalyst systems, e.g., triphenylphosphine-rhodium complex, typically produce aldehyde products with a high selectivity to linear product. Thus, such catalyst systems do not increase the availability of desirable branched-chain aldehyde products.
Other rhodium-based hydroformylation catalyst systems, e.g., tricyclohexylphosphine-rhodium complex or dicyclohexylphenylphosphine-rhodium complex, produce aldehyde product mixtures with very low linear to branched chain isomer ratios. Indeed, such catalyst systems frequently have lower selectivity to linear product than do the high pressure cobalt-based catalyst systems. Therefore, a catalyst system capable of operating at low pressure while producing linear to branched-chain product ratios comparable to the ratios obtained with high pressure cobalt-based catalyst systems would be highly desirable. Such a catalyst system would allow a commercial aldehyde producer to shift from an expensive high pressure process based on a cobalt catalyst system to a much less expensive low pressure process based on a rhodium catalyst system.